1. Field of the Invention
This invention relates to an information signal recording apparatus which employs a rotary magnetic head apparatus such as a video tape recorder.
2. Description of the Related Art
In video tape recorders, a Video signal is recorded in oblique magnetic tracks on a magnetic tape by means of a rotary magnetic head apparatus. However, in digital video tape recorders, since the amount of data to be recorded is great, generally a single digital video signal is divided into a plurality of channels and recorded in independent tracks on a magnetic tape for the respective channels.
FIGS. 7 to 9A to 9C illustrate an example of a manner in which a digital video signal is recorded divisionally in four A to D channels while a digital audio signal is recorded divisionally in a plurality of channels.
Referring first to FIG. 7, there is shown a rotary magnetic head apparatus which includes magnetic heads 1A to 1D. The magnetic heads 1A and 1B are located on a head drum 2 in an angularly spaced relationship by a predetermined angle .theta. and in a vertically offset relationship from each other while the other heads 1C and 1D are located on the head drum 2 similarly in an angularly spaced relationship by the predetermined angle .theta. and in a vertical offset relationship from each other. The magnetic heads 1A and 1B and the magnetic heads 1C and 1D are located in an angularly spaced relationship by 180 degrees from each other. The heads 1A and 1C and the heads 1B and 1D have opposite azimuth angles relative to each other.
The rotary magnetic head apparatus including the heads 1A to 1D is rotated in a field frequency in synchronism with an input video signal while a magnetic tape 3 is fed at a fixed speed obliquely along a circumferential face of the rotary magnetic head apparatus over an angular range greater than 180 degrees.
Referring also to FIGS. 8A to 8D, tracking signals STA and STB are formed at or around a starting point of time of the former half TFR of each field period, and tracking signals STC and STD are formed at or around a starting point of the latter half TBK of each field period. The tracking signals STA to STD are supplied to the heads 1A to 1D, respectively. It is to be noted that the signals STA to STD are, for example, alternating signals which have a fixed level at a fixed frequency and have different frequencies from one another.
Meanwhile, an input stereo audio signal is converted from an analog signal into a digital signal and divided into four A to D channels, and encoding processing for error correction, time base compression and delaying processing and modulation processing for recording are performed for the digital audio signals of the A to D channels so as to make, for example, digital audio signals SSA to SSD at the positions following the signals STA to STD, respectively, as seen from FIG. 8B. The signals SSA to SSD are supplied to the heads 1A to 1D, respectively.
Further, an input video signal is converted from an analog signal into a digital signal and divided into four A to D channels, and encoding processing for error correction, time base compression and delaying processing and modulation processing for recording are performed for the video signals of the A to D channels so as to make for example, digital audio signals SVA to SVD at the positions following the signals SSA to SSD as seen from FIG. 8C. The signals SVA to SVD are supplied to the head 1A to 1D, respectively.
As a result, such signals as seen in FIG. 8D are supplied to the heads 1A to 1D, and a set of tracks 4 are formed for each one field period as seen, for example, from FIG. 9B.
In particular, first, during the former half TFR of a one field period, the heads 1A and 1B scan the tape 2 obliquely and signals STA, SSA, SVA and STB, SSB, SVB are supplied to the heads 1A and 1B, respectively. Consequently, as shown in FIG. 9A, the signals STA and STB are recorded into track areas 4TA and 4TB at the top positions of two tracks 4, respectively, and then the signals SSA and SSB are recorded into track areas 4SA and 4SB at the positions following the track areas 4TA and 4TB of the two tracks 4, respectively. Finally, the signals SVA and SVB are recorded into track areas 4VA and 4VB at the positions following the track areas 4SA and 4SB of the two tracks 4, respectively.
Then, during the latter half TBK of the one field period, the heads 1C and 1D scan the tape 3 obliquely and signals STC, SSC, SVC and STD, SSD, SVD are supplied to the heads 1C and 1D, respectively. Consequently, as seen in FIG. 9B, the signals STC and STD are recorded into track areas 4TC and 4TD at the top positions of next two tracks 4, respectively, and then the signals SSC and SSD are recorded into track areas 4SC and 4SD at the positions following the track areas 4TC and 4TD of the tracks 4, respectively. Finally, the signals SVC and SVD are recorded into track areas 4VC and 4VD at the positions following the track areas 4SC and 4SD of the tracks 4, respectively.
Then, similar recording is performed also during a next field period so that next four tracks 4 are formed as seen in FIG. 9C, and thereafter, a video signal and an audio signal are recorded onto a set of tracks 4 for each one field period in a similar manner.
It is to be noted that, in this instance, the angular distance .theta. and the offset between the heads 1A and 1C and between the heads 1B and 1D and the feeding speed of the tape 3 are set in advance so that adjacent track areas (tracks) may be formed in a contiguous relationship to each other.
In this manner, a digital video signal can be digitally recorded by dividing it, for example, into four A to D channels in this manner.
In this instance, for example, the track areas 4TA, 4SA and 4VA are portions of a single track formed by single scanning of the head 1A, and the widths and the positions of them in the widthwise direction are equal to each other. This similarly applies to all of the other track areas.
Accordingly, upon reproduction, the heads 1A to 1D can scan the tracks 4 regularly by performing tracking servoing using the track areas 4TA to 4TD.
Further, upon editing, by recording a new video signal or audio signal with reference to the track areas 4TA to 4TD for tracking, the positions of the track areas 4VA to 4VD or 4SA to 4SD to be formed in response to the new video signal or audio signal can be made to coincide with the original track areas 4VA to 4VD or 4SA to 4SD.
Accordingly, upon editing, it does not occur that the new track areas 4SA to 4SD or 4VA to 4VD are overwritten with part of the original track areas 4VA to 4VD or 4SA to 4SD or on the contrary the original areas 4VA to 4VD or 4SA to 4SD remain without being erased.
Further, even if editing is performed, since it does not occur that overwriting takes place with a necessary track area or an original track area remains unerased, the non-recorded portions between the track areas 4SA to 4SD and 4VA to 4VD, that is, guard section, can be reduced sufficiently, thereby enhancing the rate of utilization of the tape 3.
It is to be noted that, in the following description, when it is not necessary to distinguish the track areas 4TA, 4SA and 4VA from each other, they are represented by the track 4A, and also the other track areas are represented by the track 4B, 4C or 4D.
By the way, electronic editing of the tape 3 is divided into two types including insert editing and assemble editing, and both types of editing involves the following processing:
(1) The tape 3 is rewound, for example, for three seconds from an editing start point. PA1 (2) Reproduction of the tape 3 is started from the thus rewound point. In this instance, the reproduction is performed in synchronism with a video signal to be recorded newly as a result of editing. PA1 (3) After the tape 3 reaches the editing start point, the video tape recorder is changed over from the reproduction mode to the recording mode.
In short, if such processing is performed, then no disorder of the arrangement of the tracks 4 takes place at the editing start point, and accordingly, a reproduced screen or reproduced sound is not disordered at the editing start point.
Actually, however, if editing of a tape is performed where such servo control as described above is involved, a trouble sometimes occurs with a result of such editing.
In particular, as shown in FIG. 10, ideally the track widths WTA to WTD of the tracks 4A to 4D are equal to a designed value WT0.
Actually, however, when the heads 1A to 1D are mounted onto the head drum 2, some errors occur with the offsets HHB and HHC of the heads 1B and 1C from the mounting reference plane. As a result, upon recording, an edge portion of the track 4B formed by the head 1B is erased by the following or trailing head 1C so that the track width WTB of the track 4B is reduced from the designed value WT0.
If the tolerances (allowable values of errors) of the offsets HHB and HHC are, for example, .+-.1.5 .mu.m with respect to the designed values of them, then, in the worst case, that is, when an error of 1.5 .mu.m occurs in the offset increasing direction with the offset HHB and another error of 1.5 .mu.m occurs in the offset decreasing direction with the offset HHC, the track width WTB of the track area 4TB is reduced by 3 .mu.m (=1.5 .mu.m+1.5 .mu.m) from the designed value WT0 for the track area 4TB.
Further, in a digital video tape recorder, in order to assure a high recording density, the azimuth angles of the heads 1A to 1D are made so different as described above so that no guard band may be formed in the tracks 4 as seen in FIGS. 9A to 9C. To this end, for example, as shown in FIG. 12, actually the track widths WHA to WHD of the heads 1A to 1D are made greater than the track widths WTA to WTD of the tracks 4A to 4D so that edge portions of tracks formed by preceding or leading heads may be erased by following or trailing heads to adjust the track widths WTA to WTD of the tracks 4A to 4D to the designed value WT0.
Thus, it is assumed that the tolerance of the track width WHB of the head 1B (and heads 1A, 1C and 1D) is, for example, .+-.1 .mu.m with respect to the designed value WH0, that is, EQU WHB=WH0.+-.1 .mu.m (1)
Also it is assumed that the designed value WT0 of the track width WTB of the track area 4TB is ##EQU1##
Consequently, from the equation (2) above, EQU WHO=WT0+1 .mu.m (3)
Since the tolerance of .+-.1 .mu.m is allowed for the track width WHB of the head 1B, the (actual) track width WHB of the head 1B is given, from the equations (1) and (3) above, by ##EQU2## This similarly applies to the track widths WHA, WHC and WHD of the other heads 1A, 1C and 1D.
Further, it is assumed that, from errors of the offsets HHB and HHC, for example, as shown in FIG. 13 (the tracks 4B and 4C are same as in FIG. 11), the track area 4TB is formed with a width WTB smaller by 3 .mu.m than the designed value WT0, that is, with the width WTB given by EQU WTB=WT0-3 .mu.m (4)
It is also assumed that the track width WHB of the head 1B of another video tape recorder which is used to edit the tape 3 has an error of +1 .mu.m which is the upper limit of the tolerance, that is, it is assumed that the track width WHB is given by EQU WHB=WHO+1 .mu.m (5)
Consequently, the difference .DELTA. between the track width WHB of the head 1B and the track width WTB of the track area 4TB is given by EQU .DELTA.=WHB-WTB
and substituting the equations (4) and (5) above into the equation give just above, ##EQU3## is obtained.
Then, substituting the equation (2) into the equation (6), ##EQU4## is obtained.
Therefore, in the worst case, the track width WHB of the head 1B of the video tape recorder used for for editing is greater by 5 .mu.m than the track width WTB of the track area 4TB of the tape 3. And, the width of 5 .mu.m makes an insensitive zone in tracking servoing, and accordingly, tracking servoing becomes indefinite. In particular, if, for example, as shown in FIG. 14, the head 1B is within the range of (A) to (B) and (B) to (C) of FIG. 14 with respect to the track area 4TB, then it is considered that tracking is effective, and .+-.2.5 .mu.m is the tracking accuracy then.
To a digital video tape recorder in which the track widths WTA to WTD of the tracks 4 are reduced so as to assure a high recording density, the value of .+-.2.5 .mu.m in tracking accuracy is a considerably dissatisfactory value.
Besides, the errors of the track widths WTA to WTD of the tracks 4 are frequently increased by a temperature variation of the offsets HMA to HMD of the heads 1A to 1D, and in this instance, the difference value .DELTA. is further increased, resulting in further degradation of the tracking accuracy.
If editing is performed while the tracking accuracy is degraded in this manner, a reproduced screen or reproduced sound will be disordered at a point where editing was started.